1. Field of the Invention
The present invention relates to a semiconductor laser device and a method of manufacturing the same, and more particularly to a semiconductor laser device for use in information electronic apparatuses, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, a DVD device has increased in demand as one of high-speed and large-capacity storage devices. The DVD device includes therein a DVD-R/RW device and a CD-R/RW device in a hybrid form. Therefore, the DVD device uses two kinds of semiconductor lasers, i.e., a semiconductor laser (a laser diode having an emission wavelength of about 650 nm (hereinafter, the laser diode is referred to as “an LD”) for the DVD device, and a semiconductor laser (an LD having an emission wavelength of about 780 nm) for the CD device.
A monolithic two-wavelength semiconductor diode (hereinafter referred to as a monolithic two-wavelength LD) has been developed. In the monolithic two-wavelength LD, such two semiconductor lasers for emitting laser beams having two wavelengths are not formed by forming a plurality of single semiconductor lasers for emitting laser beams having respective specific wavelengths to assemble them, but mounted on the same substrate on one chip. Thus, the alignment for the emission point positions of the semiconductor lasers for emitting laser beams having different emission wavelengths can be precisely performed. Also, it is possible to reduce the number of components or parts of an optical system.
A method of manufacturing the monolithic two-wavelength LD will hereinafter be schematically described.
Firstly, an n-type AlGaInP cladding layer (hereinafter “n-type”, “p-type”, and a conductivity type free from the added impurity are represented by “n-”, “p-”, and “i-”, respectively),a multiple quantum well (hereinafter referred to as “an MQW”) active layer including a barrier layer and a well layer, are epitaxially grown on an n-GaAs substrate by using the MOCVD method or MBF method. Then, a first p-AlGaInP cladding layer, a first etching stop layer (hereinafter “an etching stop layers ” is referred to as “an ESL layer”) made of Ga0.58In0.42P for example, and a second p-AlGaInP cladding layer are successively laminated on the MQW active layer through the epitaxial growth by using the MOCVD method or MBF method. Thereafter, a stripe-like resist pattern is formed so as to cover the second p-AlGaInP cladding layer. Then, the n-AlGaInP cladding layer, the MQW active layer, the first p-AlGaInP cladding layer, the first ESL layer, and the second p-AlGaInP cladding layer are selectively etched away until the n-GaAs substrate is exposed with the stripe-like resist pattern used as an etching mask. As a result, the n-AlGaInP cladding layer, the MQW active layer, the first p-AlGaInP cladding layer, the first ESL layer, and the second p-AlGaInP cladding layer are left in the form of a first mesa-like lamination structure. The first mesa-like lamination structure forms a semiconductor laser for a CD device.
Next, the resist pattern left on the surface of the first mesa-like lamination structure is removed. Thus, an n-AlGaInP cladding layer, an MQW active layer, a first p-AlGaInP cladding layer, a second ESL layer made of (Al0.5Ga0.5)0.51 In0.49P, and a second p-AlGaInP cladding layer are successively formed on the GaAs substrate including the first mesa-like lamination structure. Thereafter, a stripe-like resist pattern is formed so as to cover the second p-AlGaInP cladding layer adjacent to the first mesa-like lamination structure. Then, the n-AlGaInP cladding layer, the MQW active layer, the first p-AlGaInP cladding layer, the second ESL layer, and the second p-AlGaInP cladding layer are selectively etched away until the second p-AlGaInP cladding layer of the first mesa-like lamination structure is exposed with the resist pattern used as an etching mask. As a result, the n-AlGaInP cladding layer, the MQW active layer, the first p-AlGaInP cladding layer, the second ESL layer, and the second p-AlGaInP cladding layer are left in the form of a second mesa-like lamination structure. The second mesa-like lamination structure thus formed forms a semiconductor laser for a DVD device.
Next, a resist pattern for formation of ridge waveguides is formed on the surfaces of the second p-AlGaInP cladding layers of the first and second mesa-like lamination structures. The dry etching is individually performed until the etching is stopped at each of the first and second ESL layers with the resist pattern used as an etching mask. Thus, ridge waveguides are formed in the first and second mesa-like lamination structures, respectively.
Alternatively, instead of forming the first and second ESL layers in the first and second mesa-like lamination structures, respectively, a resist pattern for formation of the ridge waveguides are formed on the surfaces of the second p-AlGaInP cladding layers of the first and second mesa-like lamination structures, respectively. Then, the dry etching is performed with the resist pattern used as an etching mask, and stopped in accordance with time control, thereby forming the ridge waveguides.
The controllability for the processing for the ridge waveguide exerts an influence on the size precision of the etching depth of the formed ridge waveguide. The size precision of the ridge waveguide exerts a large influence on an FFPx (“an FFP” is an abbreviation of “a Far Field Pattern”) as a transverse spreading angle of a laser beam, and thus exerts a large influence on the emission characteristics of the laser element. Hence, this etching process is an important process. Consequently, during the processing of the ridge waveguide, the controllability for the etching is enhanced by providing an etching stopper layer in the epitaxial growth layer.
Examples of the monolithic two-wavelength LD having the etching stopper layer formed in the epitaxial growth layer are described as below. That is to say, there is known a two-wavelength laser device having a first laser diode and a second laser diode. In this case, the first laser diode having an emission wavelength of 780 nm includes a first p-type cladding layer made of Al0.4Ga0.6As, a p-type etching stop layer which is made of GaInP and which is laminated on the first p-type cladding layer, and a second p-type ridge-like cladding layer which is made of Al0.4Ga0.6As and which is formed on the p-type etching stop layer. Also, the second laser diode having an emission wavelength of 650 nm includes a first p-type cladding layer made of (Al0.7Ga0.3)0.5In0.5P, a p-type etching stop layer which is made of GaInP and which is laminated on the first p-type cladding layer, and a second p-type ridge-like cladding layer which is made of (Al0.7Ga0.3)0.5In0.5P and which is formed on the p-type etching stop layer. In addition, there is known a two-wavelength laser device in which each of the etching stop layers of the first and second laser diodes of the two-wavelength laser diode described above is made of AlGaAs. These two-wavelength laser diodes, for example, are disclosed in Japanese Patent Laid-open No. 2002-261397 (paragraphs [0043], [0045] and [0066], and FIGS. 1 and 7).
In addition, there is known a technique relating to mesa etching in forming a stripe (mesa portion) of a compound semiconductor layer including an active layer in manufacture of a semiconductor laser element. In the semiconductor laser element to which the technique described above relates, an etching end point detecting layer made of GaAlAs is formed on an n-GaAs substrate. Also, an isolation layer made of an n-type GaAs, an n-type cladding layer made of GaAlAs, an active layer made of GaAlAs, a p-type cladding layer made of GaAlAs, and a cap layer made of GaAs are successively formed on the etching end point detecting layer. Then, the dry etching is performed with a stripe-like insulating film formed on the cap layer used as an etching mask. Al which is exposed at a time point when the etching end point detecting layer is etched away is detected in the form of an emission spectrum, thereby stopping the dry etching. As a result, this method makes it possible to etch the mesa portion with higher precision in processing size than the time management is performed during the etching. This technique, for example, is disclosed in Japanese Patent Laid-open No. Shou 64-73726 (a top left column and a top right column of page 109).
A method of stopping etching when the etching for the ridge waveguide is performed is normally as below. That is to say, the ESL layer is provided in the epitaxial growth layer in order to stop the etching by using the chemical property of the wet etching liquid. Alternatively, the time control is performed through the dry etching.
However, when the time control is performed during the dry etching, the precision of the etching depth is insufficient.
In addition, the ESL layer used when the wet etching is performed is largely different in material constitution from the layer to be etched. For this reason, when the ESL layer is left in the semiconductor laser element, the electrical and optical characteristics of the semiconductor laser element may be impaired depending on the circumstances.
In particular, in the case of the monolithic two-wavelength LD, when the ESL layer is left in the LD for the DVD device, the oscillation efficiency is reduced due to the light absorption, which may become one of causes of impeding the high output promotion.
In addition, even in the case where the etching end detecting layer is provided in order to stop the dry etching during dry etching process, when the etching end detecting layer is left in the semiconductor laser element, the electrical and optical characteristics of the semiconductor laser elements may be impaired under certain circumstances. For this reason, it is necessary to adopt a structure such that even if left in the semiconductor laser element, the etching end detecting layer has no influence on the electrical and optical characteristics of the semiconductor laser diode as much as possible.
In addition, even if the etching end detecting layer is used, in particular, in the case of the monolithic two-wavelength LD, when the etching end point detecting layer is left in the LD for the DVD device, the oscillation efficiency is reduced due to the light absorption, which causes an impediment in the high output promotion.